The present invention relates to a process for the production of xylose from a paper-grade, hardwood pulp. More specifically, this invention relates to a process wherein the xylan contained in said pulp is extracted using an aqueous solution of a xylanase enzyme. Optionally, the process also comprises one or two alkalic treatments. Xylose is obtained by a hydrolysis of the xylan exctracted from the pulp. The paper-grade hardwood pulp used as raw material is preferably soda pulp or kraft pulp. In one embodiment of the present invention, dissolving-grade pulp of high purity is co-produced with xylan.
Xylose is a valuable raw material in the sweets and flavouring industries, for example, and particularly as a starting material in the production of xylitol. Xylose is formed in the hydrolysis of xylan containing hemicellulose. Vegetable materials rich in xylan include wood material from wood species, particularly hardwood, such as birch, aspen and beech, various parts of grain (such as straw and husks, particularly corn and barley husks and corn cobs), bagasse, coconut shells, cottonseed skins, etc.
The dissolving-grade pulp obtained is useful in the manufacture of viscose rayon, cellulose esters such as cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. The production of these cellulose derivatives requires a source of high quality cellulose feedstock. Wood pulp, however, requires extensive purification before it is suitable for viscose or cellulose ester manufacture. The additional purification, which involves treatment with alkali to remove and destroy hemicelluloses and bleaching to remove and destroy lignin reduces the yield and increases the cost of xe2x80x9cdissolving-gradexe2x80x9d cellulose derived from wood pulp.
xe2x80x9cPulpxe2x80x9d is an aggregation of random cellulosic fibers obtained from plant fibers. As used herein, the term xe2x80x9cpulpxe2x80x9d refers to the cellulosic raw material used in the production of paper, paperboard, fiberboard, and similar manufactured products. Pulp is obtained principally from wood which has been broken down by mechanical and/or chemical action into individual fibers. Pulp may be made from either hardwoods (angiosperms) or softwoods (conifers or gymnosperms). Hardwood and softwood pulps differ in both the amount and the chemical composition of the hemicelluloses which they contain. In hardwoods, the principal hemicellulose (25-35%) is glucuronoxylan while softwoods contain chiefly glucomannan (25-30%) (Douglas W. Reeve, Pulp and Paper Manufacture, Vol. 5, pp. 393-396).
There are three general types of chemical pulps:
(1) Soda pulp is produced by digesting wood chips at elevated temperatures with aqueous sodium carbonate.
(2) Kraft pulp is produced by digesting wood chips at temperatures above about 120xc2x0 C. with a solution of sodium hydroxide and sodium sulfide. Some kraft pulping is also done in which the sodium sulfide is augmented by oxygen or anthraquinone. Although kraft pulping removes most of the lignin originally present in the wood, enough remains that a bleaching step is required to give pulp of acceptable color. As compared with soda pulping, kraft pulping is particularly useful for pulping of softwoods, which contain a higher percentage of lignin than do hardwoods.
(3) Sulfite pulp is produced by digesting wood with sulfur dioxide and an alkali such as calcium, magnesium, or sodium base. The process operates in the presence of a good deal of free sulfur dioxide, at low pH. Although this process, like kraft pulping, separates most of the lignin from the cellulose fibers, considerable color remains.
xe2x80x9cDissolving-grade pulpxe2x80x9d is pulp which has been purified sufficiently for use in the production of viscose rayon, cellulose ethers, or cellulose esters with organic or inorganic acids. It may be produced from either kraft, soda, or sulfite pulp by bleaching and other treatments which will be discussed herein. Historically, dissolving-grade pulp (in contrast to paper-grade pulp) referred to pulp which reacted with carbon disulfide to afford a solution of cellulose xanthate which then could be spun into fibers (viscose rayon) with evolution of carbon disulfide and regeneration of cellulose. Dissolving-grade pulp now refers to pulp which is used to manufacture various cellulose derivatives such as inorganic and organic esters, ether, rayon and the like.
xe2x80x9cBleachingxe2x80x9d is the removal of color from pulp, primarily the removal of traces of lignin which remains bound to the fiber after the primary pulping operation. Bleaching usually involves treatment with oxidizing agents, such as oxygen, peroxide, chlorine, or chlorine dioxide. Classically, the pulp is treated with chlorine, then extracted with caustic, and finally treated with hypochlorite. The alkaline extraction may be with either hot or cold caustic. The relative merits of extraction with cold, versus hot, caustic are discussed at length by M. Weyman in The Bleaching of Pulp, W. Howard Rapson, editor, TAPPI Monograph Series No. 27 (1963), Technical Association of the Pulp and Paper Industry, New York, N.Y., Chapter 5, pp. 67-103. Weyman concludes that cold caustic extraction is the superior method for xylan removal from pulp.
While the chief purpose of the chlorine and caustic treatments is to render the residual lignin in the pulp soluble and extractable, the caustic also degrades and dissolves not only a substantial amount of the hemicellulose, but it also attacks the cellulose itself, with resulting decreases in degree of polymerization and pulp yield. The low molecular weight of some of the hemicellulose fragments makes them hard to isolate, while in some cases (prehydrolysis kraft), the harsh conditions convert the hemicelluloses to decomposition products. In conventional operation, therefore, no attempt is made to recover useful products from the hemicellulose. Chlorine bleaches lead to undesired impurities and make recycle of caustic very difficult. The use of chlorine as the bleaching agent also inevitably produces traces of extremely toxic chlorinated dioxins.
One measure of the effectiveness of bleaching is the brightness of the resulting pulp. Brightness is defined as the reflectivity of a specified standard surface using blue light with a peak wavelength at 457 nm.
Hardwood pulp produced by the kraft process contains a significant amount of hemicelluloses, chiefly xylans. The xylans, in moderate amounts, are desirable in paper manufacture because they help maintain a random dispersion of fiber in the furnish, resulting in more uniform and mechanically stronger paper webs. However, when pulp is used in the production of cellulose esters and other cellulose derivatives, the pulp normally must contain a very low level of xylan. Hardwood kraft pulp for paper manufacture generally contains about 80 to 84% cellulose, about 15 to 20% xylans, and about 0.3-3% mannans. In contrast, dissolving-grade pulp suitable for cellulose ester manufacture for fiber and film applications should contain about 97 to 98.5 weight percent cellulose, not more than about 3 weight percent, e.g., 0.5 to 3 weight percent, xylans, and not more than about 0.5 weight percent, e.g., 0.1 to 0.5 weight percent, mannans. This requirement for higher purity necessitates more drastic treatment with alkali, with resulting decrease in pulp yield. Since the hemicelluloses removed normally are not recovered from such treatments, they are used, if at all, as fuel and have negligible value. The manufacture of dissolving pulps is discussed in detail by J. F. Hinck et al., Chapter VIII, Dissolving Pulp Manufacture in Volume 4, Sulfite Science and Technology of Pulp and Paper Manufacture, Third Edition, O. V. Ingruber, M. J. Kocurek, and A. Wong, ed., published by the Technical Section, Canadian Pulp and Paper Association Montreal, QC, Canada, pp. 213-243. Although the relative amounts of impurities vary somewhat between kraft and sulfite pulps, both contain significant amounts of lignin and hemicelluloses which must be reduced.
U.S. Pat. No. 4,008,285 (and related U.S. Pat. No. 4,075,406) contains a brief review of early attempts to produce xylose from natural products such as wood. The ""285 patent also describes a process for purifying the pentosan-rich solution obtained by acid hydrolysis of xylan-containing raw material. The process involves first purifying the hydrolysate by ion exclusion and color removal, then subjecting the purified solution to chromatographic fractionation. The recovery of the pulp by-product is not disclosed.
U.S. Pat. No. 4,087,316 describes a process for removing cellulosic fibers from seed hulls, such as cottonseed, and for obtaining xylose by hydrolysis from the remaining hull fragments in the presence of dilute sulfuric acid. The resulting xylose hydrolysate may be hydrogenated to xylitol.
U.S. Pat. No. 4,742,814 discloses a process for obtaining xylitol and, optionally, cellulose and lignin from lignocellulose vegetable materials by treatment with a mixture of water and lower aliphatic alcohols and/or ketones at elevated temperature and pressure followed by separation of fibrous materials, organic solvents, and lignin from the treatment solutions. The oligosaccharides and polysaccharides remaining in solution from this process are hydrolyzed by dilute acid.
U.S. Pat. No. 5,084,104 is concerned with recovery of xylose from hydrolysates of such natural materials as birch wood, corn cobs, cotton seed hulls, etc. The disclosed process involves subjecting the hydrolysate to a chromatographic column comprising a strong anion exchange resin, and eluting a xylose-rich fraction. No reference is made to the recovery of any cellulose remaining after extraction of the xylose.
A more recent article (Gernot Gamerith and Hans Strutzenberger, Xylans and Xylanases, J. Visser et al., ed., (1992), pp. (339-348)) discusses the recovery of xylan during viscose pulp purification. Suggested uses are as a raw material for such products as furfural, xylitol, xylose, etc. In the process disclosed, beech-wood pulp produced by magnesium bisulfite cooking is first bleached with alkaline peroxide and hypochlorite, which reduces the xylan content to about 3.6%. This pulp is then treated with xe2x80x9chigh concentratedxe2x80x9d sodium hydroxide solution to reduce the xylan content sufficiently for the pulp to be used in viscose production. Xylan is recovered by acidification of the caustic solution. An unspecified amount of xylan remains in the final pulp which, apparently, is sufficiently pure for use in viscose rayon production. Although no pulp yields are given, the rather drastic alkali treatment suggests that the process resulted in a substantial loss of cellulose.
Bleaching is another step in pulp production. Conventional bleaching processes involving chlorine and alkali present environmental problems as mentioned above, and they also substantially reduce the amount of dissolving-grade pulp which can be recovered from wood. Some work has been done to determine whether the xylans in wood pulp can be hydrolyzed and removed by the action of enzymes. Most prior work has been concerned merely with sufficient removal of xylan to free residual lignin which is bound to the fibers, and aid in pulp bleaching. A number of articles and reviews have been published which deal with this aspect of the use of enzymes in pulping. A review, Enzymatic Treatment of Pulps by Thomas W. Jeffries in Emerging Technologies for Materials and Chemicals from Biomass, Roger M. Rowell, Tor P. Schulz, and Ramani Narayan, ed.; Advances in Chemistry Series No. 476 (1992), pp. 322-327 discusses pulp bleaching with hemicellulases. A recent article (L. P. Christov and B. A. Prior, Enzyme and Microbial Technology, 18, 244-250 (1996)) describes the use of repeated, alternating, treatments with the hemicellulases derived from the yeast, Aureobasidium pullulans and alkali to enchance bleaching of sulfite pulps. The production of xylose is not discussed in these articles.
The following U.S. patents disclose the use of enzymes as an aid in pulp bleaching. None of these patents discloses the production and/or recovery of chemical-grade, or dissolving-grade pulp, and none discusses the recovery of xylose.
U.S. Pat. No. 5,457,046 discloses enzymes with xylanolytic activity.
U.S. Pat. No. 5,407.827 discloses pulp bleaching by means of delignification using thermostable xylanase.
U.S. Pat. No. 5,395,765 discloses a process for treating pulp with an enzyme to improve pulp bleachability and reduce the amount of chlorine used.
U.S. Pat. No. 5,369,024 discloses the use of xylanase for removing color from kraft wood pulps.
U.S. Pat. No. 5,179,021 discloses a pulp bleaching process comprising oxygen delignification and xylanase enzyme treatment.
U.S. Pat. No. 5,116,746 discloses that cellulase-free endoxylanase enzyme is useful in pulp delignification.
U.S. Pat. No. 5,081,027 discloses a method for producing pulp by a treatment using a microorganism and its related enzymes.
U.S. Pat. No. 2,280,307 discloses a process of manufacturing paper.
Finnish Patent 55,516 discloses a method for the production of xylan from bleached or unbleached kraftwood pulp. According to said method, the pulp is treated with aqueous alkali, and xylan is precipitated from the alkalic solution by carbon dioxide. Xylan with a high purityxe2x80x94and therefore suitable for the production of xylosexe2x80x94is said to be obtained. The xylan yields based on the xylan contained in the starting pulp are not discussed.
The mechanism by which hemicellulose-degrading enzymes (xylanases and mannanases) assist in color removal or brightening of wood pulp is not completely clear and may be complex (Saake, Clark, and Puls, Holzforschung, 49, pp. 60-68 (1995)). Internal structural changes in the pulp fibers, in addition to surface modification by hydrolysis of reprecipitated xylan from the surface of kraft fibers and loosing of the bonds between the hemicelluloses and residual lignin may also be important.
Christov and Prior, Biotechnology Letters 13, 1269-1274 (1993) describe the preparation of dissolving pulp, in contrast to paper-grade pulp, by treating bleached sulfite (not kraft) pulp with xylanases, specifically enzymes of Aureobasidium pullulans. They state that even with high enzyme loadings and 24 hour incubation periods, xylan removal was limited. The use of xylanases in pre-bleaching of bamboo kraft pulp for paper manufacture recently has been reported (Pratima Bajpai and Pramod K. Bajpai, TAPPI Journal 79(4), 225.230 (1996)).
We have found that an enzymatic treatment according to the invention improves the quality of the pulp for use as a source of xylan in that it improves the solubility of xylan whereby xylan is more easily removed from pulp. This results in increased recovery of xylan and, accordingly, higher yields of xylose. Without being bound to the theory, these improvements are believed to have their basis in the increased solubility of xylan, which is caused by the xylanase treatment.
Accordingly, an object of the present invention is to provide a process for the production of xylose from a paper-grade hardwood pulp comprising the steps of:
extracting xylan contained in said pulp into a liquid phase;
subjecting the xylan contained in the obtained liquid phase to conditions sufficient to hydrolyze xylan to xylose; and
recovering the xylose, wherein the extracting step includes at least one treatment of an aqueous suspension of said pulp or an alkali-insoluble solid material thereof with a xylanase enzyme.
In this specification and appended claims, the term xe2x80x9cxylanxe2x80x9d refers both to the native xylans present in the paper-grade hardwood pulp and to the slightly degraded products formed from these native xylans during the xylanase treatment of the present invention.
In one embodiment of the invention, the process comprises a treatment of the pulp with an aqueous solution of an alkali before the xylanase treatment. According to this embodiment, the solid material separated from the alkalic slurry is subjected to a xylanase treatment.
In another embodiment, the process of the invention comprises, instead of or in addition to the above alkalic treatment, an alkalic treatment of the solid material obtained from the liquid/solid separation after the xylanase treatment.
In the process of the present invention, either the starting pulp or the solid material obtained after the alkalic treatment of the pulp is contacted with a mixture of water and an effective amount of at least one xylanase enzyme.
In a preferred embodiment, the process of the present invention further comprises an alkalic treatment of the pulp and/or of the solid material obtained from said liquid/solid separation. Especially, the process containing an alkalic treatment both before and after the xylanase treatment results in a simultaneous production of dissolving-grade pulp of very high quality and high yields of xylose.
The xylanase enzymes preferably used in the practice of the present invention are those xylanase enzymes which are substantially free of cellulase activity, i.e., those which do not substantially degrade the cellulose content of the pulp. See, for example, the xylanase enzymes described in U.S. Pat. Nos. 5,369,024, 5,395,765 and 5,407,827 and the references disclosed in these patents. In the preferred embodiment of the present invention, which contains an alkalic treatment both before and after the xylanase treatment, such xylanase enzymes afford a cellulose product with a sufficiently low xylan content. Suitable xylanases are available from a number of sources and exhibit a wide range of activities under a variety of operating conditions. The variability of enzymes and the optimum conditions at which they are effective is further discussed by Bajpai and Bajbai, TAPPI Journal 79(4), 225-330 (1996).
In general, the enzyme treatment is carried out at a temperature between about 0 and 80xc2x0 C., preferably between about 20 and 80xc2x0 C., and most preferably between 30xc2x0 C. and 70 xc2x0 C., at a pH between 2 and 12 for a time between 0.1 and 10 hours, preferably between 0.5 and 3 hours. The pH and temperature at which an enzyme exhibits maximum activity vary substantially and are highly specific for a given enzyme. The pH and temperature at which a given enzyme is most effective can be determined readily by those skilled in the art.
The amount of xylanase enzyme required to give satisfactory results depends upon the degree of xylan removal which is desired, the reaction conditions, and the particular enzyme used. Although xylanase assay typically is expressed by enzyme manufacturers as xe2x80x9cunits/mLxe2x80x9d, the units are measured differently by different manufacturers and, consequently, the xe2x80x9cunits/mLxe2x80x9d assay is meaningful, if at all, only with respect to a specific enzyme supplied by a specific manufacturer. For a given enzyme type and source, the amount of enzyme to be used is that required to give the desired purity of dissolving grade pulp. The weight ratio of water to the solid pulp material (dry basis) during the xylanase treatment may be about 2:1 to 1000:1, preferably about 4:1 to 35:1.
The slurry obtained after the xylanase treatment is subjected to conventional liquid/solid separation wherein the solid material present in the slurry is separated, e.g. by filtration or centrifugation, from the liquid phase comprising a solution of xylanase enzyme, xylan and water.
In the alkalic treatments optionally included in the process of the present invention, the solid pulp material to be treated is contacted or digested with an aqueous sodium hydroxide solution at a temperature of about 50 to 100xc2x0 C. Said solid pulp material may be the paper-grade hardwood pulp used as the raw material or the solid material obtained after the xylanase treatment. It is an important element of this embodiment of the present invention that the entire extraction/maceration is performed at a temperature in the range of 50 to 100xc2x0 C. The concentration of the sodium hydroxide in the aqueous sodium hydroxide solution normally is about 8 to 12 weight percent, with a concentration of about 9 to 10 weight percent being preferred. The amount of paper-grade pulp typically present in the pulp/aqueous sodium hydroxide slurry is in the range of about 3 to 15, preferably about 7 to 10 weight percent based on the total weight of the slurry. A particularly unique feature of the present invention is the use of elevated temperatures, e.g. about 50 to 100xc2x0 C. during the aqueous caustic treatment. It is preferred to carry out the caustic treatment at a temperature of about 60 to 80xc2x0 C. The time required for the treatment can vary substantially depending on various factors, such as the particular pulp, sodium hydroxide concentration and temperature employed. Contact times of about 1 to 30 hours are typical although contact times in the range of about 0.1 to 1 hour normally are adequate.
The slurry obtained from the above alkalic treatment is subjected to a conventional liquid/solid separation wherein the solid material present in the slurry is separated, e.g., by filtration or centrifugation, from the liquid phase comprising a solution of sodium hydroxide, xylan and water. Residual sodium hydroxide present in the solid material is reduced or removed by washing the material with water. Normally, the material is washed, for example, either by washing the filter cake on the filter, by counter current washing or by reslurrying the solids collected in water, until the wash water has a pH of less than about 8, preferably a pH in the range of about 6 to 8. This liquid/solid separation is preferably carried out at a temperature of about 50 to 100xc2x0 C., most preferably about 60 to 80xc2x0 C. This preferred embodiment results in a co-production of dissolving-grade pulp which contains little, if any, cellulose II and, therefore, is especially useful for use in the manufacture of carboxylic acid esters of cellulose.
Xylan is recovered from the liquid phase obtained after the xylanase treatment or, in the case that an alkalic treatment follows the enzymatic treatment, from any of the liquid phases obtained after these two treatments or from a combination of these liquid phases, by known procedures. A preferred method for recovering the xylan comprises the alcohol precipitation procedure described in U.S. Pat. No. 3,935,022. In this method, one or more C1-C4 alkanols are combined with the liquid phase to precipitate the xylan from the liquid. Thus, the liquid phase is preferably combined with one or more C1-C4 alkanols to effect precipitation of xylan from the liquid and the resulting mixture is subjected to liquid/solid separation to recover xylan. The-volume of the alkanol(s) combined with the liquid of said steps to effect xylan precipitation may be in the range of about 50 to 200% of the volume of the liquid although alkanol volumes of about 80 to 120% (same basis) are more typical. Methanol and ethanol are particularly preferred alkanols. The liquid may be concentrated, e.g. by vaporization or membrane separation procedures, prior to being combined with the alkanol(s).
The alkanol-containing solution obtained from the separation of precipitated xylan may be subjected to distillation to recover the alkanol(s), and in the case that NaOH is present in said solution, also the aqueous sodium hydroxide. Thus, both the alkanol(s) and the aqueous sodium hydroxide may be used repeatedly in the process.
Alternatively, a liquid phase obtained after the xylanase treatment and subsequent alkalic treatment can be concentrated by removal of water by distillation of multiple-effect evaporation until the concentration of sodium hydroxide is about 40-50 weight percent. This concentrated solution can be treated with a C1-C4 alkanol to precipitate the xylan. About 1 volume equivalent of alkanol is required. The precipitated xylan is recovered by filtration, centrifugation, or the like, and the filtrate distilled to recover the alkanol and leave a concentrated sodium hydroxide solution which can be diluted to the desired concentration for use in the xylan extraction process.
In another variation, said liquid phase can be subjected to nanofiltration through a caustic-stable membrane which allows passage of water and sodium hydroxide but does not allow the passage of dissolved organic compounds having a molecular weight above a few hundred, e.g. xylan. This process variation produces a clean sodium hydroxide stream ready for re-use and a much smaller stream in which the xylan is highly concentrated in aqueous sodium hydroxide. The xylan in this organic-rich stream may be recovered by alkanol precipitation as described above, or by neutralization of the sodium hydroxide by the addition of a mineral acid which also precipitates the xylan.
The xylan recovered as described above is converted to xylose according to conventional procedures, e.g. by acid hydrolysis or enzymatically. Procedures for the conversion of xylan to xylose and further to xylitol, and recovery processes, are described in more detail in U.S. Pat. Nos. 4,008,356, 4,025,356, 4,075,406 and 5,084,104. An acid hydrolysis suitable for converting xylan to xylose is disclosed, for instance, in WO publication No. 96/27028.
For example, heating a slurry of xylan in water, e.g. a slurry containing from about 5 to 25 weight percent solids, in the presence of a mineral acid produces xylose. The heating is normally effected at a temperature in the range of about 70 to 150xc2x0 C., preferably at about 90 to 120xc2x0 C. Examples of suitable mineral acids include sulfuric acid, hydrochloric acid and phosphoric acid.
An enzymatic conversion of xylan to xylose can be carried out, e.g. by following the procedures described in WO publication No. 91/03566 and the litterature sources referred to therein.
Xylose can be recovered from the obtained xylan hydrolysate by known procedures. The hydrolysate may first be purified, for instance, using chromatographic methods. If acid hydrolysis is used for the conversion of xylan to xylose, the excess of anions can be removed from the hydrolysate, e.g. by precipitation. As a last step, xylose may be recovered from the hydrolysate or a purified solution obtained from it by crystallization using known methods, e.g. as disclosed in WO publication No. 96/27028.